Welcome to LOC mate.
Problem 1.
You will need to wait for a Navigator owner to help here mate, but I believe the CD-ROM disc itself or
the CD-Rom Drive in the boot is at fault. Is the disc dirty in any way or damaged ? You may well need
to purchase an updated CD-ROM disc which probably won't be cheap........
Problem 2.
The ERR3 issue has nothing at all to do with the Sat-Nav problem (other than it isn't working ).
It does however mean the OE Head Unit is ready for the bin. All is not lost however, as you can replace
the Head Unit with a Single or Double DIN aftermarket one, and get a module from E-Bay
to keep the Sat-Nav functioning, but only if you get to the bottom of problem 1 first. You will need this
module as the cars amplifier needs to be fooled into thinking the standard Head unit is still functioning
correctly to retain operation.
Alternatively you could buy a secondhand OE Head Unit with the same number by the cassette slot, but there
is no guarantee how long it would work for, and you are better off replacing it with something more modern
that will be more reliable......
I would suggest solving problem 1 first, as problem 2 is a relatively straightforward procedure.......
Janey's link above to Stevie's thread will help with lots of info.
regards
Wozza